1. Field of the Invention
The present invention relates to a thermal activation device for a heat-sensitive self-adhesive sheet and to a printer assembly employing the thermal activation device, the heat-sensitive self-adhesive sheet having a heat-sensitive adhesive layer formed on one side of a sheet-like substrate thereof and used as an affixing label, for example, the heat-sensitive adhesive layer being normally non-adhesive but developing adhesiveness when heated. Particularly, the invention relates to a technique advantageously applied to energy control of a thermal head used for thermally activating the heat-sensitive adhesive layer.
2. Description of the Related Art
Recently, many labels affixed to products for indication of bar codes, prices or the like are stored in a state where the pressure-sensitive adhesive layer is provided on a back side of a recording surface (printable surface) and has a liner (separator) temporarily affixed thereon. Unfortunately, the labels of this type require the liner to be removed from the pressure-sensitive adhesive layer when used, thus always producing waste.
As a system negating the need for the liner, there has been developed a heat-sensitive self-adhesive label having a heat-sensitive adhesive layer on a back side of a label-shaped substrate thereof, the heat-sensitive adhesive layer being normally non-adhesive but developing adhesiveness when heated. On the other hand, a thermal activation device for heating the heat-sensitive adhesive layer of the heat-sensitive self-adhesive label is now under development. For example, there is known a thermal activation device employing a thermal head as heating means.
The thermal head normally includes an array of heat generating elements (resistances) which are energized with voltage thereby generating heat. In the thermal activation device employing this thermal head, the array of heat generating elements are energized in unison by applying a predetermined voltage pulse simultaneously. The heat-sensitive self-adhesive label is thermally activated on a per-line basis as advanced in a direction orthogonal to the array of the heat generating elements, whereby the heat-sensitive self-adhesive label is caused to develop adhesive force on the overall surface thereof.
In a case where the heat-sensitive self-adhesive label is thermally activated by means of such a thermal activation device, importance is attached to the development of the adhesive force of a magnitude -to prevent easy peel-off of the heat-sensitive self-adhesive label from a support material (an article affixed with the label). Hence, it is a common practice to carry out the thermal activation in a manner that the overall adhesive surface of the heat-sensitive self-adhesive label may have a great adhesive force (of a magnitude that once affixed, the label can never be peeled off or will be broken if it is forcibly peeled).
In this case, however, such a great adhesive force to prevent the peel-off of the heat-sensitive self-adhesive label from the support material also leads to a disadvantage that when the affixed label is not needed any more, the label cannot be peeled off easily. For instance, labels for use on baggage to be checked before getting on board airplanes may desirably be peelable because these labels are usually unnecessary after the baggage is received.
It may be contemplated to control the energy for thermally activating the head-sensitive self-adhesive label, which is used for such a purpose, thereby decreasing the developed adhesive force to a point. In the case of the thermal activation device employing the thermal head, for example, the applied energy is controllable by way of the magnitude of a voltage pulse or the pulse width (voltage application time).
Unfortunately, there are some types of heat-sensitive adhesives which are difficult to control the adhesive force developed therein. As to an adhesive having a characteristic curve indicated by a solid line T1 in FIG. 9, for example, an adhesive force of at least F1 (the great adhesive force) can be readily attained by applying an energy of at least E1. However, the development of an adhesive force in the range of at least F2 to less than F1 (a small adhesive force) requires the magnitude of voltage pulses or pulse width to be so controlled as to limit the applied energy in the range of E1 to E2. Besides, a relation between the energy applied to the adhesive and the adhesive force (see, for example, T1, T2 in FIG. 9) depends upon ambient temperatures and hence, the control of the magnitude of pulse voltage or pulse width may be complicated at some ambient temperatures where the heat-sensitive self-adhesive label is used.
An alternative technique for controlling the adhesive force has been proposed wherein the heat-sensitive self-adhesive label is thermally activated at local places thereof for locally developing the great adhesive force rather than developing the adhesive force on the overall surface thereof. That is, a ratio between an area of a portion having the great adhesive force and the total area of the label is controlled thereby adjusting the degree of adhesive force on the basis of the whole area of the label (JP-A-2000-48139).
According to the above technique, however, there exists a portion having no adhesive force at all, which leads to the following problem. In a case where the portion without the adhesive force is located near an end of a label, the label is prone to be peeled so easily that the label affixed to a baggage is likely to be lost unless the baggage is handled with care. Thus, the technique is not practicable. In a case where the thermal activation is focused on circumferential edges (frame form) of a label, an area without the adhesive force occupies a central part of the label in order to decrease the adhesive force on the basis of the overall label surface and hence, the central part of the label is more susceptible to air invasion. The invaded air lifts up the label from the support material, resulting in a low-quality appearance of the label. In addition, it is a cumbersome task to produce a thermal activation pattern for indicating what area of the heat-sensitive self-adhesive sheet is to be thermally activated and what area thereof is to be left un-activated.